The present invention relates to a radial turbine used for a turbocharger or the like for a vehicle such as an automobile. More particularly, this invention relates to a variable displacement turbine having movable nozzle blades (variable nozzle).
In a variable displacement turbine used for a turbocharger or the like, a plurality of movable nozzle blades are rotatably provided between two opposed side wall surfaces around an outer periphery of a turbine blade wheel through shaft bodies, respectively. The nozzle discharge angle and the passage area of an entrance of each of the movable nozzle blades (xe2x80x9cmovable blade entrancexe2x80x9d) vary by rotation of the movable nozzle blades, and thereby the turbine capacity also varies.
The variable displacement turbine is provided with a support pin which sets the clearance extending across between the two side wall surfaces so that a distance between the two side wall surfaces between which the movable nozzle blade is disposed, i.e. a nozzle clearance, is maintained at an appropriate value even if the side surfaces are exposed to high temperature gas such as exhaust gas.
The support pin is indispensable for the structure, but since the support pin exists in the working medium flow path in the turbine entrance, the support pin becomes a resistance in the flow path. This may cause lowering of the turbine efficiency an aerodynamic influence. Especially, the aerodynamic influence of the support pin, when the quantity of flow is small, is great, and a reduction in turbine efficiency when the quantity of flow is small is a problem.
Various attempts have been made to lower the reduction of the turbine efficiency caused by the support pin, as shown in Japanese Utility Model Application Laid-open No. H7-25249, Japanese Patent Application Laid-open No. H11-190219 and Japanese Patent Application Laid-open No. H11-336554. According to the techniques described in these publications, the mechanical structure of the support pin is improved, the resistance of the flowpath caused by the support pin is reduced, and the reduction of the turbine efficiency is lowered.
The conventional technique which improves the mechanical structure of the support pin achieves the tentative object, but there is a tendency of the structure of the support pin to become complicated, and there is a limit in reducing the resistance of the flow path by just improving the mechanical structure of the support pin. In order to obtain better turbine efficiency, it is necessary to further reduce the resistance of the flow path caused by the support pin.
It is an object of the present invention to provide a variable displacement turbine capable of effectively reducing the flow path resistance caused by a support pin, capable of reducing the aerodynamic influence of the support pin when the quantity of flow is small, and capable of improving the turbine efficiency when the quantity of flow is small.
The present invention provides a variable displacement turbine in which a plurality of movable nozzle blades are rotatably provided between two opposed side wall surfaces around an outer periphery of a turbine blade wheel by respective shaft bodies. A clearance setting support pin extending across between the two side wall surfaces is provided in order to maintain a distance between the two side wall surfaces at an appropriate value. The support pin is disposed at a position in front of a front edge of the movable nozzle blade in a state in which the movable nozzle blade is located at a rating opening position (xcex1=20xc2x0xc2x15xc2x0) which is set between a maximum opening position and a minimum opening position.
With this structure, an aerodynamic effect, an especially aerodynamic effect in a state of the rating opening position being at the minimum opening position, is reduced by the disposition position of the support pin with respect to the movable nozzle blade, and efficiency on the region at a small flow rate is enhanced.
To obtain further effects in the variable displacement turbine of the invention, the support pin is disposed on an extension line or in the vicinity of the extension line of a logarithmic spiral which passes a blade thickness central portion of the movable nozzle blade located in the rating opening position (xcex1=20xc2x0xc2x15xc2x0).
To obtain further effects in the variable displacement turbine of the invention, when an outer diameter of the support pin is defined as d, the support pin is disposed in a distance range away from the front edge of the movable nozzle blade through 3d. 
To obtain further effects in the variable displacement turbine of the invention, the disposition position of the support pin is further defined such that the support pin is located on the side of a negative pressure surface of the movable nozzle blade by increasing the opening of the movable nozzle blade from the rating opening position (xcex1=20xc2x0xc2x15xc2x0), and the support pin is located on the side of a pressure surface of the movable nozzle blade by reducing the opening of the movable nozzle blade from the rating opening position (xcex1=20xc2x0xc2x15xc2x0).
The invention also provides a variable displacement turbine in which a plurality of movable nozzle blades are rotatably provided between two opposed side wall surfaces around an outer periphery of a turbine blade wheel by respective shaft bodies. A clearance setting support pin extending across between the two side wall surfaces is provided in order to maintain a distance between the two side wall surfaces at an appropriate value. The support pin is disposed at a position closer to a front edge of the movable nozzle blade than a rotation center position of the movable nozzle blade on the side of a pressure surface of the movable nozzle blade without contacting with the movable nozzle blade located at a maximum opening position.
With this structure also, the aerodynamic effect, especially the aerodynamic effect in a state of a low opening position, is reduced by the disposition position of the support pin with respect to the movable nozzle blade, and the efficiency in the region of a small flow rate is enhanced.
To obtain further effects, the disposition position of the support pin is further defined such that the support pin is between a front edge position of the movable nozzle blade and a substantially intermediate position between a rotation center of the movable nozzle blade and the front edge of the movable nozzle blade.